The use of electrochemical processes in extractive metallurgy is constantly growing because of the relative stability of electrical energy costs and an increasing application of hydrometallurgical processes for environmental reasons. Electrowinning and electro-refining processes using plate electrodes have been in operation for many decades. However, such processes suffer from capacity limitations. The cell production capacity per unit volume is limited not only by the chemical problems of polarization and the nature of the cathode deposit, but also by the practical problem of electrode surface area per cell unit volume.
Recently, considerable interest has been focused on electrolytic systems using particles that can act either as an extension of the electrode with which they are in contact or behave as an electrode independently, thus achieving a large increase in electrode area per unit cell volume. Such electrodes are particularly useful for processes that can be carried out economically at low current densities, such as electrodeposition of metals from dilute solutions.
One type of electrode is that referred to in the art as a fluid bed electrode (FBE). The term comes from the observation that when a bed of electrically conductive material is fluidized by an upward flow of electrolyte, the bed can be made to function as an electrode by inserting a conductor in the bed (e.g., a cathode) through which an electric current is passed. It is known to employ agitated bed electrodes where the particles are kept in suspension by stirring or by suspension electrodes where the particles are agitated by a vibrating plate or diaphragm. Static beds have been employed; however, such beds tend to cement together during metal deposition and lose their efficiencies.
A fluid bed electrode design that has been proposed is one having a plane parallel configuration comprising a bed of electrically conductive particulate material with an anode disposed above it, the bed being supported on a porous support. In this configuration, the electrolyte is passed through the porous bed support and flows out of the cell at the top. The current feeder for the cathode is located in the bed and is perpendicular to the flow direction of the electrolyte. The flow of current in this type cell is parallel to electrolyte flow.
A disadvantage of this cell using fine nickel powder of approximately 90 microns average size as a cathode is the tendency toward hydrogen gas evolution on the surface of the nickel particles which caused flotation of the particles to the anode surface which in effect caused a change in the position of the anode with respect to the bed. The over-expansion of the bed adversely affected the electrochemical properties of the fluid bed. In the parallel cell configuration, it was difficult to maintain a uniform flow distribution of the electrolyte because of the large cross-sectional area to height ratio of the bed.
Even when the fluidization of the cathode bed was satisfactory, plating tended to occur preferentially on the bed surface facing the anode. Because the cathode feeder was disposed at the bottom, bed heights of only a few centimeters could be tolerated.
Side-by-side electrodes have been proposed comprising a fluid bed electrode with a second electrode inserted into the bed, the second electrode being coated with an insulating material, e.g., polypropylene, of sufficient porosity to provide current flow while avoiding shorting of the cell. Various embodiments of side-by-side electrodes are disclosed in the literature.
In an article entitled "A Preliminary Investigation of Fluidized Bed Electrodes" by J. R. Backhurst et al (Journal of the Electrochemical Society [Electrochemical Technology]; November 1969, pp. 1600-1607), a cell with a side-by-side electrode is disclosed for use in the cathodic reduction of the organic compound nitro benzene sulfonic acid to metanilic acid in aqueous sulfuric acid, a typical cell comprising a cathode bed of copper powder in a cathode chamber isolated by a porous diaphragm which in turn is surrounded by an annular anode (e.g., a lead anode) to provide a cell having a concentric configuration. In cathodically reducing the organic compound, copper-coated glass particles of 450 to 520 micron size were employed, the fluidized bed volume ranging from about 5% to 25% greater than the static bed volume.
In a paper entitled "Feasibility Study On The Electrowinning of Copper With Fluidized-Bed Electrodes" by J. A. E. Wilkinson et al (Institute of Mining and Metallurgy [London]; September, 1972, Vol. 81, pp. C157-C162), a fluidized-bed electrode is disclosed for the electrowinning of copper from leach liquors and other solutions. A side-by-side configuration proposed comprised anode and cathode compartments separated by a non-porous ion exchange membrane, the cathode comprising the fluidized bed. The results indicated that copper could be deposited from dilute solutions.
Another paper of interest is one entitled "The Fluidized Bed in Extractive Metallurgy" by D. S. Flett (Chemistry and Industry; Dec. 16, 1972, #24, pp. 983-988). In this paper, a side-by-side electrode configuration is disclosed comprising a vertical cell in which a fluidized bed is supported vertically on one side of the cell by a membrane and in which a vertically disposed anode is spaced to one side of the membrane-supported fluid bed. The electrolyte is fed from a leach circuit to the fluidized cathode cell for the recovery of metal values therefrom.
A number of cell configurations are considered in the paper entitled "Feasibility Study On The Electrowinning of Copper With Fluidized-Bed Electrodes" by J. A. E. Wilkinson et al (Institute of Mining and Metallurgy [London]; Vol. 82, pp. C119-C125, 1973). One arrangement comprises a side-by-side electrode configuration formed of concentric anode and cathode compartments. In this configuration, the cathode feeder which is tubular is embedded in the bed such that part of the bed is shielded from the anode which is not desirable.
In U.S. Pat. Nos. 3,941,669 and 3,951,773 assigned to Noranda Mines Limited, a fluidized bed electrode system is disclosed comprising an electrode chamber having a porous bed, a main electrode consisting of a bed of electrically conductive particles, an auxiliary electrode (i.e., an anode) with the surface thereof partially impregnated with a synthetic organic fiber screen cloth so that the auxiliary electrode can be introduced into the fluidized bed without risk of forming a short circuit, a current feeder extending into the bed, means for feeding a fluidizing gas into the bed of particles, and means for feeding solution through the fluidized bed.
U.S. Pat. No. 3,988,221 discloses the use of a cathodic fluid bed for the electrolytic removing of heavy metal ions from solution. The cell employed uses a centrally disposed anode with a permeable coating thereon so that the anode can penetrate the fluid bed without shorting the cell.
A problem encountered in fluid bed electrolysis is the formation of gas bubbles during electrolysis (e.g., the formation of oxygen at the anode in the anode chamber and the formation of hydrogen at the cathode in the cathode chamber) which can have an adverse effect on the fluid bed process unless the gases are removed during circulation of the electrolyte. For example, the rapid release of oxygen in the anode chamber may be sufficient to force the electrolyte out of the anode chamber and affect the efficiency of the cell.
It would be desirable to provide a fluid bed electrode system which overcomes the aforementioned disadvantages and which would be particularly desirable for recovering nickel from laterite leach solutions which normally contain less than about 10 grams per liter (gpl) of nickel and up to about 1 gpl cobalt. It would also be desirable to provide a fluid bed electrolyte system for effecting separation of nickel and cobalt from fairly concentrated nickel solutions, for scavenging nickel and cobalt from ion exchange feed solutions, or even to remove impurities from nickel leach solutions, such as Cu, Cd, Zn, and the like.